Acid strength is measured by the position of equilibrium of ionization in water. In other words acid strength is the function of the stability of conjugate base of the acid. More is the stability of conjugate base of the acid, more is the acidity of acid.
Which of the following is weakest acid?

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds is the weakest acid, we need to analyze the stability of their conjugate bases and the nature of the compounds provided. The compounds in question are:

1. Phenol
2. Para-hydroxybenzoic acid
3. Meta-hydroxybenzoic acid
4. Ortho-hydroxybenzoic acid

### Step-by-step Solution:

**Step 1: Identify the nature of the compounds.**
- Phenol is an aromatic compound with a hydroxyl group (-OH) attached to a benzene ring.
- Para-hydroxybenzoic acid, meta-hydroxybenzoic acid, and ortho-hydroxybenzoic acid are all carboxylic acids with a hydroxyl group attached to a benzene ring.

**Hint:** Recognize the functional groups in each compound to understand their acidic nature.

**Step 2: Understand the concept of acidity.**
- The strength of an acid is determined by its ability to donate protons (H⁺) and the stability of its conjugate base after deprotonation.
- A more stable conjugate base corresponds to a stronger acid.

**Hint:** Remember that the stability of the conjugate base is key to determining acid strength.

**Step 3: Analyze the conjugate bases.**
- When phenol loses a proton, it forms the phenoxide ion, which is relatively stable due to resonance.
- The carboxylic acids (para-hydroxybenzoic acid, meta-hydroxybenzoic acid, and ortho-hydroxybenzoic acid) will form carboxylate ions upon deprotonation. These carboxylate ions are stabilized by resonance and the electronegative oxygen atoms.

**Hint:** Compare the resonance stabilization of the conjugate bases formed from each acid.

**Step 4: Compare the acidity of the compounds.**
- The carboxylic acids are generally stronger acids than phenol due to the greater stability of their conjugate bases (carboxylate ions).
- Among the three carboxylic acids, the presence of the hydroxyl group can influence acidity. However, all three carboxylic acids will still be stronger than phenol.

**Hint:** Recall that carboxylic acids typically have higher acidity than alcohols.

**Step 5: Conclusion.**
- Since phenol is not a carboxylic acid and has a less stable conjugate base compared to the carboxylic acids, it is the weakest acid among the four compounds listed.

**Final Answer:** The weakest acid among the given compounds is **Phenol**.